Men with indwelling catheters and men and women with suprapubic catheters were studied in their homes. Urine and blood were cultured and body temperature recorded after every catheter change. Nearly all patients had infected urine after 4 weeks of catheterisation, and all had bacteriuria after longer periods, usually with a mixture of organisms. Culture on selective media revealed a wider range of organisms than was detected on routine C.L.E.D. and blood agar with antibiotic sensitivity disks, but routine culture gave adequate information for clinical purposes. Bacteraemia was demonstrated after 20 of 197 changes of urethral catheter and after one of 19 changes of suprapubic catheter; but no patient had pyrexia or other symptoms. However, two had rigors on other occasions. When assessing "risk factors" for blood-stream infection in catheterised patients, it is important to record the total incidence of bacteraemia, asymptomatic as well as symptomatic.